1. Field of the Invention
The present invention relates to an oxide etch process which is highly selective to nitride and, in particular, to an oxide etch process based upon C.sub.4 F.sub.8 or C.sub.2 F.sub.6 that exhibits high selectivity to nitride and which can be used on surfaces of uneven topology.
2. Description of the Related Art
A significant challenge in semiconductor fabrication is to etch silicon oxide in the presence of silicon nitride without also etching the nitride, i.e., while maintaining a high selectivity to the nitride. For example, in the case of an oxide layer located over a nitride layer, since both the oxide and nitride materials generally etch at the same rate in a typical fluorocarbon etch plasma, a process of providing additional selectivity to the nitride must be found.
When a fluorine-substituted hydrocarbon, such as C.sub.3 F.sub.8, is used as an etchant, the fluorocarbon radicals react in the plasma to form a passivating coating of carbon-fluorine polymer which forms over the materials being etched. However, this polymer is dissociated by oxide atoms formed during the etch of the exposed oxide portions. Thus, as the silicon oxide continues to etch, the exposed silicon nitride portions etch at a much slower rate due to the presence of the passivating coating. However, the passivating layer is also attacked by free fluorine atoms present in the plasma and, thus, the nitride also continues to be etched. As a result, a selectivity over about 8:1 of silicon oxide to silicon nitride is not achievable with such prior art etch processes due to the presence of free fluorine atoms in the plasma. Since state of the art devices having submicron dimensions require selectivity of over 10:1, and even 30:1, an etch process for etching oxide in preference to nitride with a selectivity of over 10:1 is highly desirable.
Commonly-assigned U.S. Pat. No. 5,423,945 describes the provision of a scavenger for fluorine, such as a source of silicon or carbon, which, when used in combination with fluorine-substituted hydrocarbon etch gases, results in the formation of a carbon-rich polymer which does not dissociate over nitride surfaces. This result is apparently due to either the reduced free fluorine content in the plasma, or the reduced fluorine content in the polymer, or both. In any event, use of a scavenger for fluorine in combination with fluorine-substituted hydrocarbon etch gases results in an oxide etch having a selectivity to nitride of over 10:1, and as high as approaching infinity (i.e., no measurable nitride loss).
More recently, an additional problem has been discovered in the case where at least the nitride surfaces of the nitride/oxide structure being etched are not flat, as for example, the sidewalls of a slot or raised steps such as, for example, nitride-coated polysilicon lines.
This type of structure is illustrated in FIG. 1 wherein raised polysilicon lines 10 and 12, formed over a substrate 2, are coated with a conformal layer 20 of nitride, over which is formed an oxide layer 30 and a photoresist mask 40. When oxide layer 30 is etched, through mask opening 42 in photoresist mask 40, down to conformal nitride layer 20, nitride portions 22 on the sidewalls of raised polysilicon lines 10 and 12 are also at least partially etched, indicating that the above-described protective polymer is either not forming on the generally vertical surfaces (surfaces generally perpendicular to, or at least not planar with, the underlying substrate 2), or the protective polymer is being more readily attacked by the etchant gases on the non-planar surfaces than are the corresponding polymer portions formed on horizontal surfaces (surfaces generally planar to the underlying substrate 2) such as nitride portion 26 between raised polysilicon lines 10 and 12.
Commonly-assigned U.S. Pat. No. 5,423,945 discloses that the addition of one or more hydrogen-containing gases, preferably one or more hydrofluorocarbon gases, to one or more fluorine-substituted hydrocarbon etch gases in contact with a scavenger for fluorine, in a plasma etch process for etching oxide in preference to nitride, results in a high selectivity to nitride which is preserved regardless of the topography of the nitride portions of the substrate surface. Preferably, one or more oxygen-bearing gases are also added to reduce the overall rate of polymer deposition on the chamber surfaces and on the surfaces to be etched. The application discloses examples of processes based upon C.sub.3 F.sub.8, CH.sub.3 F and CO etch chemistries.